New type of free air ionization chamber



Jan. 14, 1964 F. H. ATTIX 3,118,064

NEW TYPE OF FREE AIR IONIZATION CHAMBER 2 Sheets-Shem: 1

Filed Aug. 30, 1961 INVENTOR FRANK H. ATTI X BY @W/ ATTORNEY Jan. 14, 1964 F. H. ATTIX 3,118,064

mzw TYPE OF FREE AIR IONIZATION CHAMBER Filed Aug. 30, 1961 2 Sheets-Sheer. 2

INVENTOR FRANK H. ATTIX BY m ATTORNEY United States Patent 3,118,064 NEW TYPE OF FREE AER IONIZATTON CHAR'IBER Frank H. Attila, 5125 27th Ave. SE, Killer-est Heights, Md. Filed Aug. 3%, 1961, fier. No. 135,083 8 Claims. (Cl. 253-835) (Granted under Title 35, US. Code (1952}, see. 266) This invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention is directed to free-air ionization chambers and more particularly to a new type of free-air ionization chamber.

The roentgen has been used for many years as a unit of X-ray exposure dose. The unit of X-ray exposure dose is defined as follows: One roentgen is an exposure dose of X- or gamma radiation such that the associated corpuscular emission per 0.001293 gram of air produces, in air, ions carrying one electrostatic unit of quantity of electricity of either sign. Thus a measure of the number of roentgens requires that one measure all of the ionization produced in air by the high-speed electrons that are themselves produced within the defined mass of air. The free-air ionization chamber has been used in the experimental realization of this uni Heretofore a number of diiferent prior art designs have been used as the standard for determining roentgen exposure dose each of which have their drawbacks. Such prior art is described in Design of Free-Air ionization Chambers, H. O. Wyckoli and F. H. Attix, Handbook 64, US. Department of Commerce National Bureau of Standards, issued December 13, 1957. These prior art devices depend upon guarding-electrodes to produce a uniform electric field. The guarding-electrode system usually consists of a complicated configuration of metal plates and/or wires, which must be fabricated and assembled with great precision to ensure uniformity of the electric field within the chamber. Each electrode must be accurately biased at the proper electrical potential necessary to give rise to field uniformity. This is usually accomplished in prior art devices by means of connections to a voltage divider consisting of a series of accurately known resistors, connected to a high voltage power sup ply (eg. 2090 to 5000 volts). This supply must thus be capable of delivering considerable electrical current (e.g. 1 milliamp) which passes through the voltage divider to ground. It must also maintain an extremely stable and constant voltage in order not to interfere with measurements of the ionization current, ordinarily performed with an electrometer. Such a power supply is an expensive accessory to the prior art free air chamber, because it must maintain a constant output voltage while supplying a relatively large current.

The present invention overcomes these drawbacks of the prior art free-air ionization chambers by eliminating the necessity for having a uniform electric field, thus eliminating the special guarding electrode network and its accompanying voltage divider and high-powered voltage supply. Errors resulting from field-nonuniformity, either due to inaccuracies in electrode configuration, or in the components of the voltage divider, we thus eliminated.

It is therefore an object of the present invention to provide a free-air ionization chamber which is simple in construction and use and more accurate than prior art free-air chambers.

Another object is to provide a free-air ionization charnber which avoids the need for electric field uniformity and therefore needs no guarding-electrodes to produce such a field.

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Still another object is to provide a new free-air ionization chamber which requires fewer parts and eliminates errors arising in prior art devices.

Other objects and advantages of the present invention will hereinafter become more fully apparent from the following description of the annexed drawings wherein:

FIG. 1 is a side view with the relative sections in a collapsed position;

FIG. 2 is a side view of the device with the relative parts in an expanded position; and

FIGS. 3, 4 and 5 are modifications of the device illustrated in FIGS. 1 and 2.

The present invention is directed to a free-air chamber which is designed to have an extensible length. A collimated X-ray or gamma ray beam enters through a window in the chamber wall at one end along the axis of the chamber and leaves the chamber at the opposite and through another window. The chamber is ordinarily operated with air under ambient conditions of air pres sure and temperature. However, the chamber may be placed within an enclosure to allow its use with other filling gases and/ or pressures and/ or temperatures. Ionization produced by X-rays or gamma rays interacting with the gaseous medium on the inside in the preferred embodiment is collected on a rod-shaped electrode positioned off the axis of the chamber and operated at a different potential from the outer shell of the chamber. Other electrode arrangements may be used as shown by the modification shown by FIGS. 4 and 5. The ionization is measured with at least two different chamber lengths (the minimum length be ng large enough to ensure that electrons originating at the fixed central plane will not reach any part of the chamber wall) and the difierence in ionizations is a measure of the absolute exposure dose at a specified point in the X-ray or gamma ray beam passing through the chamber.

Now referring to the drawings, there is shown in a side view, a general configuration of a free-air ionization chamber 19 made in accordance with the invention. The chamber may be made with two telescoping halves l1 and 12 or as a cylinder-and-piston or any other configuration which achieves the same result of an extensible free-air chamber which may be circular in cross section as shown or any other shape which will provide the same result. However in order for the instrument to be capable of absolute measurements of exposure dose, the inner radius of the chamber shell should be sufficiently large to accommodate the range of secondary electrons which are generated by the X- or gamma-ray interactions with air inside the chamber. The interior surfaces of the two ends 13 and 14 may be covered by a layer of air equivalent conducting material 15 such as graphite or aquadag coated lucite, having a thickness at least equal to the maximum range of any secondary electrons generated by the X-ray energies for which the chamber is designed. The structural material of the cylindrical wall surface is made of a fairly low atomic number material such as aluminum, so that the ionization due to scattered X-ray interactions will not be enhanced by the photoelectric effect generated by X-rays scattered by the aperture in the fixed diaphragm or the air in the chamber. Windows 16 and 17 are positioned at the axis of the chamber and are formed by cutting out the end wall structure but not the air-equivalent lining 15 on the ends of the chamber to form entrance and exit ports for the X-ray beam. The air-equivalent material covering the ports 16 and 17 serves to exclude electrons which originate outside the chamber and acts as a source of secondary electrons to replace those lost into the chamber ends. At sufliciently low X-ray quantum energies (eg. below 50 kev.), where the secondary electrons are projected predominantly in directions lateral to the beam, rather than forward, the

air equivalent liners are no longer necessary, and may be omitted.

A conducting rod (e.g. aluminum) 21 is connected to the end surface of one section (ll) by an insulating member 22 and extends parallel to the axis of the chamber, off the center of the chamber, through the chamber and passes through an insulating bushing 23 in the end wall of section 12. The rod has sufiicient length that the telescoping chamber sections can be moved relative to each other to increase the inner volume of the chamber by at least a factor of two. The end of the rod projecting from the back end extends into a grounded electrostatic shield 24 which is mechanically (but not electrically) connected to the back end Wall by a grounded shield 25 which surrounds the insulated bush ng. The front insulator to which the rod is connected is also surrounded by a grounded guard ring 26 to eliminate leakage of current across the insulator to the rod. The electrostatic shield contains a sliding contact 27 which makes contact with the rod at all times, and is electrically connected to any type well known electrometer 28. The guard rings extend into the chamber a short distance along the rod to intercept any gas-multiplied ionization occurring in the higi field region immediately adjacent to the collecting rod insulators.

Instead of a separate rod being used as a collector, a sector of the chamber wall, either at the sides or on the end, may serve as the ionization-collecting electrode, as shown for example in FIGS. 4- and which will be described in detail later.

A fixed diaphragm 31 having a beam-defining aperture 32 is positioned between the X-ray source and the freeair ionization chamber to collimate the X-ray beam which is directed through the entrance window of the chamber. The X-ray beam causes ionization of the air in the chamber as the beam passes through the chamber. A high voltage is applied to the entire chamber wall (front, back and sides) to maintain a potential difference between the collector rod and the walls of the chamber and is operated at a sutliciently high voltage such that substantially all of the ionization produced in the chamber by the X-ray beam is collected on the aluminum rod. The rod is sufficiently large in diameter to avoid gas-multiplication of the ionization. The ionization current collected on the aluminum rod is measured by the electrometer 28 through the slide contact 27 within the grounded electrostatic shield 2 In operation, to use the free-air ionization chamber as a standard for the measurement of exposure dose of X-rays or gamma rays, at least two measurements of the ionization current are required. The first measurement is made with the chamber in a collapsed position as shown by FIG. 1. All of the ionization produced in the chamber is first collected, measured, and recorded while the chamber is in the collapsed position. The chamber is then expanded by moving each section the same distance so that the mid-plane remains fixed with respect to the X-ray source and the beam-defining aperture in front of the chamber. This can be accomplished by any suitable means such as by a machine screw with opposite thread from the mid-point. The chamber is vented such that in changing from the collapsed position to the expanded position a free-flow of air is maintained in and out of the chamber as the length changes and without any appreciable changes in the air temperature or pressure within the chamber. The change in length must be sufliciently large to allow an accurate determination or" the difierence in ionization with the chamber in the collapsed position and the expanded position. An increase in the length of a factor of two is sufilcient. The difference in the ionization value in the expanded position and that in the collapsed position is the ionization generated in the air added to the chamber by changing to the expanded position. This difierence-ionization will be equal to the inte gral exposure dose in the additional volume of air which is effectively centered about the fixed central plane, and

has a length equal to the increase in chamber length. This volume is analogous to the collection region lying between the principal guard electrodes of the conventional free-air ionization chamber.

Alternatively, the chamber ionization can be measured for several different length-settings, and an average value thus established for the increase in ionization per unit increase in chamber length (or volume). From this the exposure-dose can again be derived. Again in this case, the minimum chamber length used in the exposure-dose measurement should be sufiiciently great that secondary electrons originating at the fixed central plane cannot reach any part of the chamber wall.

The modification shown by illustration in FIGS. 4 and 5 illustrate other means for collecting the ionization current. As shown by FIG. 4 the end wall is formed by a collector plate 34 and an electrostatic shield 35 which are insulated from the cylinder 36 by an insulator 37. The collector plate 34 forms the inner end surface of the chamber and is electrostatically protected by the shield 35. The electrometer is electrically connected to the collector plate by any suitable means. The end wall formed by the collector and electrostatic shield is slidable relative to the cylinder to change the volume of the chamber as described above. FIG. 5 illustrates the collector as a sector 41 in the wall of the cylinder 36 in which the sector is insulated from the high voltage applied to the cylinder. The sector 41 has an electrostatic shield 42 as an inner cylinder which has the closed end 43 toward the closed end 44 of the outer cylinder 36, and insulated from the outer cylinder 36. The cylinder 42 is similar to cylinder 12 illustrated in FIG. 1 except the cylinder 42 is turned with the open end outward rather than inward. The cylinder 42 is sufliciently long that the ionization collector sector 41 is protected in all positions of movement for the two cylinders 36 and 42.

The modification illustrated in FIG. 3 is functional the same as the ionization chamber illustrated in FIGS. 1 and 2. In the modification, as shown, the chamber is formed by a cylinder 51 and a piston 52. The cylinder walls are longer in length than the walls of the telescoping halves 11 and Hot the device illustrated in FIGS. 1 and 2. The rod 21 passes through the piston and has the same electrostatic shield and ionization current connec-. tion as shown in FIG. 1. The piston is moved into and out of the cylinder as the cylinder is moved in order to keep the piston head and the end wall surface 53 of the cylinder the same distance from the center line. By use of a device with a piston, the chamber can be completely collapsed rather than just half-way as noted from the device of FIGS. 1 and 2. This will enable one to meas me the ionization for small chamber extensions, which is useful for X-ray energies lower than the maximum for which the chamber is designed. Also, the piston is simpler in construction than the telescopic half-sections.

The internal dimensions of the chamber are dictated by the X-ray energies for which the chamber is to be used. For a chamber covering the X-ray voltage range of from 5025O kv. (constant potential), a diameter of 30 centimeters is appropriate to eliminate nearly all escape of electrons into the wall. The length of the chamber in its collapsed state should also be at least equal to the maximum electron range to insure that electrons originating at the fixed central plane of the chamber cannot reach the front or back walls. For higher energy radiation, a length-to-diameter ratio greater than one may be used. For energies less than 50 kv. a diameter of only four centimeters would be required.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A free-air ionization chamber which comprises two a mechanically separate sections, on! movable Within the other, both sections of the chamber being biased at a high voltage relative to ground, an ionization-collection rod connected at one end to a wall of one section and insulated therefrom, said collection rod extending through said charnber oil the axis and parallel thereto, said rod extending through a Wall of the other section and insulated therefrom, and electrical means contacting said rod on the outside of sad chamber and indicating the amount of ionization current in said chamber.

2. A free-air ionization chamber for determining radiation exposure dose which comprises a chamber having two telescopic sections, an ionization collection rod passing through said sections off the axis thereof and parallel thereto, said ionization collection rod being insulated from the other section and passing through an end wall thereof, an electrical brush in contact with said rod and an electrometer connected with said electrical brush for recording ionization current collected on said ionization collection rod.

3. A free-air ionization chamber for determining radiation exposure dose which comprises a chamber made of two telescopic sections, an ionization collection rod connected at one end to a Wall of one section and insulated therefrom, said collection rod extending through said chamber off the axis of the chamber and parallel thereto and extending through one Wall of the other section and insulated therefrom, an electrical contact on the outside of said chamber in electrical contact with said rod, said electrical Contact directing ionization current collected by said rod to an electrometer which indicates the amount of ionization current which is proportional to the radiation exposure dose.

4. A free-air ionization chamber for determining radiation exposure dose which comprises a chamber formed of two telescopic sections, an ionization collection rod secured at one end to a Wall of one section and insulated therefrom, said collection rod passing through said chamber off the axis thereof and parallel to the axis, said collection rod extending through a wall of the other section and insulated therefrom, a grounded electrostatic shield secured about said collection rod that protrudes from said chamber, an electrical contact within said electrostatic shield and in contact with said collection rod, said electrical contact electrically connected with an electrometer to indicate the amount of ionization current which is proportional to the radiation exposure dose.

5. A free-air ionization chamber for determining radiation exposure dose which comprises a chamber formed of tWo separate relative movable sections, one of said sections fitting into the other section to form said chamber therebetween, an axially aligned Window in each of said sections for passing radiation through said chamber along the axis thereof, an ionization collection rod secured at one end to an end Wall of one section at a point oi the axis of said chamber and insulated therefrom, said rod extending through said chamber parallel to the axis thereof and extending through an end wall of the other section and insulated therefrom, a grounded guard ring positioned about said rod at each Wall, a grounded electrostatic shield secured about said collection rod that protrudes from said chamber, an electrical contact within said electrostatic shield and in contact with said collection rod, said electrical contact electrically connected with an electro neter to indicate the amount of ionization current which is proportional to the radiation exposure dose.

6. A free-air ionization chamber as claimed in claim 4 wherein the end Walls of each section of the chamber are covered with an air equivalent linin 7. A method of determining radiation exposure dose which comprises passing an X-ray beam through an air chamber to ionize the air therein, collecting and recording the amount of ionization current, increasing or decreasing the volume of said chamber and passing said X- ray beam through said chamber with the increased or decreased volume in said chamber, collecting and recording the amount of ionization current with increased or decreased volume, determining the difference between each recorded ionization current, said difference being proportional to the radiation exposure dose.

8. A method of determining radiation exposure dose by use of a free-air ionization chamber which comprises passing an ionizing radiation beam through an air chamber to ionize the air therein, collecting the ionizing current and recording the same, linearly expanding or contracting said chamber by various amounts to increase or decrease the volume or air therein, passing said ionizing radiation through said expanded chamber, collecting the ionization current and recording the amount of same, and determining any change in ionization per unit change in length, which is a measure of the radiation exposure dose.

Reterences Qited in the file of this patent UNITED STATES PATENTS 

1. A FREE-AIR IONIZATION CHAMBER WHICH COMPRISES TWO MECHANICALLY SEPARATE SECTIONS, ONE MOVABLE WITHIN THE OTHER, BOTH SECTIONS OF THE CHAMBER BEING BIASED AT A HIGH VOLTAGE RELATIVE TO GROUND, AN IONIZATION-COLLECTION ROD CONNECTED AT ONE END TO A WALL OF ONE SECTION AND INSULATED THEREFROM, SAID COLLECTION ROD EXTENDING THROUGH 